I am unable to type some of the capital letters from my keyboard.
Keys of my keyboard works, because I am able to type that letter in small case.
This problem is always there in all types of applications, eg, geany, browser, inside guest oS etc.

What could have gone wrong?
Anybody ever face such situations?

[See that, in this post also, I was unable to type some capital letters, i, u, o etc.]

I've never seen that behavior. Sometimes shift and caps lock keys get stuck or go intermittent. I'd had that - solved by hammering shift keys til they became unstuck. It would have probably been better just to clean the key board - often keys can be pulled off and snapped back on.